The use of wireless communications has become prevalent in modern society. Wireless systems are utilized daily by business, individuals, governments, etc. to provide voice and data communication. For example, cellular phones, particular “smart phones” having advanced processing as well as communication capabilities, are in widespread use daily by persons from all walks of life.
With the proliferation of wireless communications there has been a rise in awareness of the potential for harm to human tissue from exposure to high levels of radiated energy. Accordingly, many governments have established limits on the amount of energy irradiated into a portion of the body of a user of a wireless device. The United States Federal Communications Commission (FCC) has, for example, has established a specific absorption ratio (SAR) of 1.6 milliwatts per gram (mW/g) with respect to cellular phones and other personal communications devices. Similarly, the European Union European Committee for Electrotechnical Standardization (CENELEC) has established a SAR of 2.0 mW/g in a 10 g segment for the foregoing personal communications devices. The standards imposed by such government entities not only establish the acceptable SAR value, but also proscribe the area of the human body where the SAR is to be measured. In particular, both the FCC and CENELEC require that the SAR be measured at the user's ear.
Although the currently established SAR requirements are generally easily met by communications devices implementing narrow-band antenna configurations, such narrow-band antenna configurations are often not well suited for use with respect to many modern communications devices. For example, the aforementioned smart phones generally provide for both voice and high-speed data communication, often using third-generation (3G), fourth-generation (4G), and long term evolution (LTE) communications protocols. Moreover, personal communications devices are utilized around the world, often with a particular user utilizing his/her communication device in multiple countries. Such communications are typically accommodated through the use of wide-band antenna configurations by the communications devices, such as to accommodate voice and data communications bands, communications bands of different standards and different geographic regions/countries, etc.
Unfortunately, meeting the SAR requirements imposed by one or more government entity, particularly the SAR requirements of the FCC, is problematic when wide-band antenna configurations are utilized. In particular, the wide-band antennas implemented by such communications devices in order to facilitate operations such as accommodating both voice and high-speed data, operating with protocols such as 3G, 4G, LTE, etc., and/or provide a device which is operable globally, generally provide higher SAR levels than a more narrow-band antenna. For example, transmission of a signal, using a planar monopole antenna commonly implemented in smart phones today, at 1800 MHz as measured in a 1 g soft tissue analog sample at a depth of 10 mm, in accordance with the FCC SAR standards, results in a SAR measurement of 3.4, which is well over the FCC limit of 1.6. Similarly, transmission of a signal, using a planar monopole antenna, at 1800 MHz as measured in a 10 g soft tissue analog sample at a depth of 10 mm, in accordance with the CENELEC standards, results in a SAR measurement of 1.9, which is narrowly within the CENELEC limit of 2.0.
Accordingly, manufacturers of personal communications devices, such as smart phones, have adopted designs which physically place the wide-band antennas used thereby to be located as far away from the area in which SAR measurements are made in order to assure compliance. Specifically, because the SAR is typically specified as being measured at the user's ear, manufacturers have adopted configurations in which the antennas of personal communications devices are disposed at the end of the device away from the earpiece (i.e., near the mouthpiece or microphone end of the device).
Although the foregoing technique has generally been acceptable for implementing wide-band antenna configurations in personal communications systems which meet the various SAR requirements, the solution is not without disadvantage. For example, should a need arise to implement more than one transmit antenna, such as for multiple-input multiple-output (MIMO) protocols, the additional antenna elements would be located more near to the area in which SAR measurements are made, thus likely resulting in an inability to comply with SAR requirements.
Other techniques may be considered for providing communications device configurations which provide wide-band communication support while meeting SAR requirements. However, each such alternative is likewise associated with disadvantages.
For example, the use of meta-materials has been discussed with respect to antenna configurations adapted to provide suitable SAR performance. However, meta-materials are inherently narrow-band as a result of their effectively forming a LC trap resonator. In order to provide a wide-band antenna configuration using such meta-materials, the antenna element must generally be relatively large, thereby presenting a solution which is problematic with respect to the relatively small size of personal communication devices.
As another example, the use of active circuits may be considered, whereby the operational frequency of the antenna system may be tuned as needed for transmission/reception of signals. However, many modern personal communication systems, such as smart phones, must monitor a number of different frequencies (e.g., for handoff, carrier aggregation, etc.). Accordingly, the adaptive circuits would need to switch extremely fast in order to provide the requisite operation. However, such fast switching adaptive circuits are neither inexpensive nor small, thereby providing a solution which is not well suited for personal communications devices.
Another alternative for meeting the SAR requirements may be to implement a baseband solution. For example, certain operations, such as data transmission, may be discontinued during activity in which the communication device is placed near the user's head (e.g., during a voice call) to thereby provide reduced SAR. However, such solutions are generally objectionable to the users of the communications devices as the performance of the device is lessened.